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Old 8th November 2018, 09:55 PM   #1541
Enochrome is offline Enochrome  United States
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Originally Posted by PRR View Post
> how does one determine the output power of the Mofo?

Pout = ((VDC*0.64)^2)/RL
("0.64" is 0.707 for peak-to-RMS, and 0.9 for general loss.)

(This could perhaps be 0.9VDC but a hair-rich beats any-cold.)

Total DC power = 2.5 times output power

Power making heat in heatsink = also 2.5 times Pout
(Drops to 1.5X when working FULL Sine power, but you only do that on test-bench.)

Thanks for the quick and detailed response. Most of that was beyond my comprehension, so in that I might have to focus on one of the first watt amps.

Is there a thread in the Pass Labs or DIY Audio site that is dedicated to people just starting out in electronics? If not, I think that it could possibly be a good idea.

Thanks again. I do appreciate it.
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Old 8th November 2018, 10:26 PM   #1542
Zen Mod is online now Zen Mod  Serbia
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Old 9th November 2018, 12:06 AM   #1543
PRR is offline PRR  United States
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>> Pout = ((VDC*0.64)^2)/RL
> ....that was beyond my comprehension...


To estimate power output:

Determine your supply voltage. Call it "VDC".

Multiply VDC by 0.64. (Don't ask why, yet.)

Find the Square of that number. (Multiply it by itself.)

Divide that number by the load resistance "RL".

For 19V supply and 8 Ohm load:

19V times 0.64 makes 12.16.

12.16 times 12.16 makes 148.

148 divided by 8 Ohms makes 18 Watts.

However you want to work through the other calculations. The 19 Volt supply MUST have relatively BIG current; not some old answering-machine wart. The heatsink for a larger MoFo needs to be quite large.
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Old 9th November 2018, 01:38 AM   #1544
e_fortier is offline e_fortier  Canada
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Is the Output power really 18W with a 19Vdc supply ? I thought the designer mentioned 12W.

Mind you, I haven’t measured mine and 12W vs 18W is probably hard to differentiate by ears.

Tomorrow I’m helping a friend building his 2 regular MoFo.

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Old 9th November 2018, 11:51 AM   #1545
Michael Rothacher is offline Michael Rothacher  United States
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When I quote power, I'm usually talking about how much you'll get with less than 1 or 2% THD. If you have a look at the THD vs. Power graph from the article, you can see that it makes it to about 18W with around 10% THD.
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Old 10th November 2018, 11:32 AM   #1546
Dirk1 is offline Dirk1  Germany
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I can operate the MoFO with two boards as balance amplifier.

I currently operate the L`Amp 193V and wanted to use of this then the power supply.
If that works I need only a second 193V, another transformer and the other Mofo parts is that correct.

kann ich den MoFO mit zwei Platinen als Balance Verstärker betreiben.

Ich betreibe zur Zeit die L`Amp 193V und wollte von diesem dann das Netzteil nutzen.
Wenn das funktioniert brauche ich nur noch einen zweiten 193V, einen anderen Trafo und die anderen Mofo Teile ist das richtig.
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Old 12th November 2018, 07:23 PM   #1547
Eric is online now Eric  United States
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Got my MoFo amp finished over the weekend, though I haven't yet had the opportunity to listen to it.

I made it "bread board" style with short legs and ran all of the wires below deck. I made a small, wooden T-support that goes under the base of the sink to improve the airflow a bit and the last few vanes dangle over the edge of the board.

Since the sinks are isolated from one another, the mosfets are mounted with only thermal paste. This allowed me to bump the bias up to 3A for each channel and still limit thermal rise of the sinks to 25c. Using a Dell power brick rated for 12A delivers 19.1v when both channels are driven. The chokes exhibit a thermal rise of about 9c with 3A of current through them.

Can't wait to hear it tonight! It's rather heavy with all of that metal on top!
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Last edited by Eric; 12th November 2018 at 07:27 PM.
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Old 12th November 2018, 07:30 PM   #1548
Michael Rothacher is offline Michael Rothacher  United States
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Wow, that's very spiffy.
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Old 12th November 2018, 07:47 PM   #1549
Dennis Hui is offline Dennis Hui  Canada
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Beautiful work, Eric!
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Old 12th November 2018, 08:23 PM   #1550
El Reino is offline El Reino  Finland
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Beautiful work Eric!
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